1. Field of the Invention
The present invention relates to automatic gain control systems and more particularly to a changeover circuit for providing output signals indicative of the level of an automatic gain control (A.G.C.) input signal supplied thereto.
2. Background of the Prior Art
If the amplitude of the composite video signal developed in a television receiver is allowed to vary significantly, a strong incoming signal may cause the video amplifiers of the receiver to become overloaded resulting in cross modulation and clipping of the synchronizing components of the signal, while a weak incoming signal may cause the output of the video amplifiers to be too low to provide proper picture reproduction. In addition, unwanted variations of contrast may result from a video signal which is changing in amplitude. Therefore, in all television receivers some form of automatic gain control (A.G.C.) is used to keep the detected video constant despite variations in the level of the composite radio frequency (RF) input signal. The most common system is to sample and make the synch, the tip of the composite video signal equal to a direct current (DC) reference voltage. This is normally effected by means of a gated amplifier which derives a voltage (stored on a capacitor) the level of which first gain reduces the IF amplifier as the input signal increases, the RF gain remaining unchanged. After some predetermined point, gain reduction of the IF stage ceases and the IF gain remains constant with further gain reduction being made by reducing the gain of the RF stage of the receiver. For example, U.S. Pat. No. 3,697,883, which is incorporated herein by reference, describes such a system.
The overall characteristic of the aforementioned system enables the television receiver to have the best possible signal to noise ratio at low signal levels because the RF gain is fixed and optimized for good noise performance while at high signal levels, where noise performance is not as important, overloading of the receiver and hence intermodulation problems are prevented by the gain reduction of the RF stage. However, the changeover from affecting the gain reduction of the IF stage to the gain reduction of the RF stage should not result in both the RF and RF amplifiers being acted on simultaneously for a large part of the AGC curve, or conversely there should be no gap between the two regions, i.e., IF gain reduction region and the RF gain reduction region. Both of these faults lead to large changes in AGC loop gain and difficulties in controlling the loop response to disturbances.
Some contemporary changeover systems suffer in that the point of changeover between IF and RF gain control is not well-defined and hence suffer from the aforedescribed faults. Thus, there is a need for a circuit of the type being discussed which makes the possibility of either of the aforementioned effects almost zero by reducing the changeover region to negligible proportions. Such a circuit should also fulfill the needs for being suitable to be manufactured as an integrated circuit in a large scale integrated circuit comprising a television reveiver.